Red Blood Cell Production
Red blood cell production or erythropoiesis is the process by which red blood cells are formed.
In early development, RBC production begins in the yolk sac, shifts to the liver and spleen during the 3rd month of gestation, and finally to the bone marrow in the 5th month. Once adulthood is reached, the creation of RBCs is mostly restricted to the marrow from the ends of the "long" bones-the vertebrae, ribs, and pelvis-with a little produced in the skull. The life cycle of a normal RBC is about 120 days, just four months. But in that short lifetime the RBC makes an astonishing 75,000 round trips between the lungs, heart and cells of the body. Since RBCs do not possess a nucleus, they are unable to repair or synthesize new cellular components and eventually they wear out. When that happens, most aging RBCs are pulled out of circulation by specialized white blood cells called macrophages within the liver, spleen, and lymph nodes. The macrophages engulf RBCs, "digest" them and release some of their components to be recycled within the body. As old RBCs are broken down and their components re-utilized, the bone marrow is constantly at work producing new RBCs. In a healthy human being, this is a dynamic and continuous process.